1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine, which performs a fuel injection control by using an alcohol concentration in the fuel detected by an alcohol concentration detector.
2. Description of Related Art
Recently, social needs for CO2 emission reduction and use of alternative fuels are increasing. Accordingly, a demand for a vehicle having an internal combustion engine, for which any one of gasoline, alcohol (ethanol, methanol, etc.) and alcohol blended fuel can be used, is increasing. In such a vehicle, if an alcohol concentration of fuel that is currently refueled to a fuel tank is different from an alcohol concentration of fuel that is refueled to the fuel tank last time, the alcohol concentration of fuel in the fuel tank changes. A stoichiometric air-fuel ratio of gasoline is different from that of alcohol. Therefore, when the alcohol concentration of fuel changes, the stoichiometric air-fuel ratio of the fuel changes. Accordingly, it is necessary to change fuel injection quantity (actual air-fuel ratio) in accordance with the alcohol concentration of the fuel.
In this regard, JP3-085347A describes a fuel supply control apparatus that detects an alcohol concentration of fuel supplied to an internal combustion engine with an alcohol concentration sensor, and corrects fuel injection quantity in accordance with a detection value of the alcohol concentration.
With regard to the alcohol concentration sensor, JP5-507561T describes a capacitive alcohol concentration sensor that measures a capacitance of fuel in order to detect an alcohol concentration of the fuel. JP5-223733A describes an optical alcohol concentration sensor that measures a transmitted light that has transmitted through fuel in order to detect an alcohol concentration of the fuel.
The boiling point of alcohol is lower than the boiling point of gasoline. Therefore, when alcohol fuel (pure alcohol fuel or blended fuel that contains alcohol) is used, vapor (bubbles) can be generated in the fuel more easily. In the above-mentioned capacitive alcohol concentration sensor and optical alcohol concentration sensor, an accuracy of measurement of the capacitance of fuel or the transmitted light that has transmitted through fuel is lowered when vapor is generated in the fuel. Accordingly, the detection accuracy of the alcohol concentration is lowered.
In the fuel supply control apparatus described in JP3-085347A, the fuel injection quantity is corrected in accordance with the detection value of the alcohol concentration in the fuel, which is detected by the alcohol concentration sensor. Therefore, if vapor is generated in the fuel to lower the detection accuracy of the alcohol concentration sensor, the fuel injection quantity is corrected with the detection value of the alcohol concentration, which contains a large detection error. Accordingly, an accuracy of the fuel injection control is lowered, and a deviation of the fuel injection quantity (deviation of the air-fuel ratio) is increased. As a result, a rotational speed of the internal combustion engine can be unstable, and emission can become worse.